Abstract: By using the geometric analysis of grand potential, the molar grand potential-effective chemical potential curve of solution with magnetic transition can be obtained from the relevant molar free energy diagram, and it is shown that the grand potential and effective chemical potential can be separated into paramagnetic and magnetic terms. In addition. the CVM has been combined with the present magnetic transition free energy model such as M .Hillert and T.Nishizawa's models to calculate phase equilibria between solutions with magnetic transition, and this combined method has been applied to the α/γ phase equilibrium in the Fe-Mn binary system and found to be feasible.